HLA class II histocompatibility antigen (HLA-DR) binds peptides derived from antigens that access the endocytic route of antigen presenting cells (APC) and presents them on the cell surface for recognition by CD4 T-cells. HLA-DR is a heterodimer of an alpha and a beta subunit; also referred as MHC class II molecule. The peptides presented by HLA-DR and other MHC class II molecules are generated mostly by degradation of proteins that access the endocytic route, where they are processed by lysosomal proteases and other hydrolases.